Gas discharge display panels are provided with circuitry for producing a sustain voltage which is applied to each of the discharge cells in the display panel. The sustain voltage causes selected panel areas to discharge due to current avalanche within the cell at a rate determined by the sustain voltage frequency. In this manner, the selected panel area has the appearance of being continuously illuminated.
Various problems are associated with driving large gas discharge display panels. On large displays, the gas avalanche current produced by the sustain voltages can become prohibitively large. These currents are drawn from a power supply to the display panel through parasitic inductances of the cabling and ground returns. The large avalanche current caused by the simultaneous sustain operation in each of the discharge cells produces a large time rate of change of current (di/dt) through these parasitic inductances to produce a voltage across the inductances. This voltage drop produces a "notching" and ringing of the voltage across the panel as illustrated in FIG. 1. This degradation of the waveform will increase the minimum and decrease the maximum sustain voltage applied to the panel, thus reducing the operating margins. The large voltage drops and high frequency currents combine to produce electromagnetic interference and compatibility problems. Noise problems are caused by both conducted noise in the grounding systems and radiated noise from the cables.
One possible approach to eliminate these problems would be to divide some of the current paths among a plurality of independent sustain voltage circuits. The use of a plurality of independent sustainers reduces notching to a certain extent but does not completely eliminate the problem. This technique also does not solve the conducted noise problems because it does not divide up the ground return paths. Another possible technique would be to design the panel itself to draw less avalanche current. Reducing the magnitude of the currents, however, also reduces the brightness and the operating margins of the panel.
An approach to a related problem of cross-talk between panel cells is addressed in U.S. Pat. No. 3,851,211 to Greeson, Jr., which teaches a gas panel sustain sequence which drives alternating lines during one sustain sequence and a second set of alternating lines during a second sequence to thereby reduce the cross-talk problem. This technique incidentally lowers the power consumption of the driver circuits. The patent to Greeson, Jr. does not, however, relate to a staggered sustain technique for a large display panel.